Stable lead anodes



Nov 8, 1966 E. J. PARS! ET AL 3,284,333

STABLE LEAD ANODES Filed May 22, 1962 PEG. 2

ENVENTQRS Edwrdm a; Pam WWW fizazw Jr.

ATTQRNEY 3,284,333 STABLE LEAD ANODES Edgardo 3. Parsi, Wellesley Hiils, and Albert Szczur, 312, New Bedford, Mass, assignors to Ionics, incorporated, Cambridge, Mass, a corporation of Massachusetts Filed May 22, 1962, Ser. No. 196,758 2 Claims. (Cl. 2il4292) This invention relates to anodes as articles of manufacture, the method of making the same, and their use in electrolytic systems. More particularly, it is concerned with a method for fabricating anodes comprising more than one metal component, but one component being selected from the platinum group. Specifically, it relates to bielectrodes to be used as non-corroding anodes in the electrolysis of electrolytic solutions containing chloride ions. More specifically, it relates to the use of lead base metal anodes in causticclilorine cells.

In electrochemical processes such as electrolysis, electrodialysis, and in the field of cathodic protection, it is a matter of prime importance-especially where the anode comes in contact with a solution containing chloride ions-to properly select the materials from which to manufacture such anodes. In these processes there are only a few-known materials which may effectively constitute the anode because most materials, while acting as an anode, are susceptible to intense corrosion by oxygen and chlorine gas which are evolved at said anode. If only chemical characteristics of the metal were to be considered in the selection of a suitable anodic material, the metals of the platinum group would be the universal choice because of their high resistance to corrosion. However, the high cost of these precious metals prohibits their extended commercial use in most processes. In recent years much effort has been directed to the construction of anodes for use in electrolytic processes for coating the surface of an electrolytic valve metal (such as tantalum or titanium) with a precious metal like platinum. Al though the use of such anodes for commercial use has shown possibility, it is still quite expensive to fabricate them and difficulty has been encountered in producing a precious metal coating which will adhere to the base metals with sufilcient tenacity. From time to time many other materials have been tried as a substitute for the precious metals, but the primary anodic material employed todayespecia-lly in the causticchlorine industry-is still graphite. Many disadvantages accompany the use of graphite anodes since the material undergoes continual disintegration and must be replaced rather frequently; this causes interruption in the electrochemical process which is a costly operation. Also in the manu: iacture of chlorine gas by electrolysis of chloride salt solutions, the gas becomes contaminated with traces of carbon dioxide which results from the anodic oxidation of the graphite.

A continuous search has been made for an anode material which has the desirable anti-corrosive characteristics of the platinum metals, but which lacks their prohibitive cost. It has long been known that load can be used as an inexpensive and satisfiactory anode especially when employed in a sulfuric acid medium. During the anodic process the lead surface of the anode is oxidized to lead peroxide (PbO which is brownish in color and which possesses chemical stability with high electronic conductivity. Thus, a P-bO /Pb system makes an ideal anode provided the covering of lead peroxide remains in contact with the underlying lead and provided it is reformed when a discontinuity occurs in the per-oxide coating. However, where such an anode is used in the presence of chloride ions, the lead peroxide film cannot be maintained on the entire lead surface; hence, the chlonited States l atent G Patented Nov. 8, 1966 ride ions penetrate the peroxide film and form a nonconducting layer of lead chloride on the underlying lead base. This causes the lead peroxide to become insulated from said lead base. Under these conditions it has been found that at a constant current density, the voltage drop increases rapidly, and eventually the anode completely disintegrates.

Heretofore it was known that the anodic corrosion of lead in a chloride electrolyte could be prevented by introducing a single platinum wire into the surface of a one cm. lead anode (Anodic Behavior of a Lead-Platinum Biel-ectrode in Chloride Electrolytes, E. J. Lit-tauer and L. L. Shreir, Platinum Metals Review, 1961). When this bielectrode becomes anodic in a medium containing chloride ions, a film of lead chloride is first produced. However, it is rapidly replaced with stable lead peroxide which initially forms at the interface of the lead and platinum mic-roelectrode. The platinum wire, penetrating the surface of the lead anode, acts as a nucleus for the formation of lead peroxide in the vicinity of said platinum wire. When the peroxide completely covers the comparatively small anode surface, the only anodic reaction taking place is the oxidation of chloride ions to chlorine gas with the peroxide-coated base metal remaining stable.

An important advantage of the present invention over the prior art is the use of small discrete particles of noble metal (in preference to the relatively large platinum wire of the prior art). The discrete particles will give better protection since many more microelectrodes are present to act as nuclei for more rapid and complete coverage by the per-oxide coating. Additionally, the platinum wire method of protection, as shown in the prior art, may be effective when employed with extremely small anodes of 1 square centimeter or less, but proves inefiective when used in conjunction with larger, conventional electrodes of, for example, \1 square foot or more. This disadvantage has been effectively overcome by the present invention.

it is therefore the object of this invention to provide an improved and more simple method for manufacturing bimetallic anodes of any size which are stable in electrolytic processesespecially those involving chloride-containing solutions.

Another object is to make an improved bimetallic anode which conprises numerous microelectrodes dispersed throughout the lead anode, or only on the surface. I A further object is to achieve these results in a simple, rapid, and economical manner.

Further objects and various advantages of this invention will be apparent from a study of this disclosure, the drawings, and appended claims:

FIGURE 1 is a preferred embodiment of the invention and represents a perspective view of a fully-formed bielectrode anode constructed in accordance with the present invention; and

FIGURE 2 is a side elevational view of the bielectrode in cross section taken in a plane represented by the line 2-2 of FIGURE 1.

The drawings illustrate an anode in sheet form :1 which is comprised of a lead base metal 5 having imbedded therein noble metal particles 6 and 4. The upper portion of the drawings represents the active working area of the anode which comprises a solid adherent layer or coating of lead peroxide 3 and surface microelectrodes 6 (finely divided noble metal particles). Below the coated area are sub-surfiace microelectrodes 4' of a noble metal embedded within the lead base metal 5. It can be appreciated that a 'bielectrode could be constructed which would contain only surface microelectrodes since it would only be necessary for the required protection to have noble metal particles on the anode surface. The manner of producing the above-illustrated bielectrode types is hereinafter more fully described.

The term platinum metal as used herein means the precious metals in Group VIII of the Periodic Table; that is, the term includes the following metals; ruthenium, rhodium, palladium, iridiuum, platinum, osmium, and alloys of these metals. Platinum, however, will be referred to hereinafter as representative, and as the preferred metal of Group III, for the purposes of this invention. It is also intended that the term lead as used herein include the commercial form containing a wide range of minor impurities. The present invention is also applicable to electrodes having base metals comprising alloys of lead and antimony as well as lead and silver.

The process of this invention involves imbedding numerous discrete particles of a noble metal, or a combination of noble metals, on the surface and/or throughout the lead anode. However, where the bielectrodes contains only surface microelectrodes, it is possible for these to flake ofi or to become dislodged during operation. This would result in a lead anode having no further protection; thus eventual distintergration would occur. However, this does not occur in a bielectrode constructed with noble metal particles throughout since the deeper imbedded platinum particles would eventually become exposed to the surface and again act as protective microelectrodes, preventing further corrosion of the base metal.

During the manufacture of the bielectrode, the platinum particles are added to molten lead-the temperature of which is kept below the melting point of platinum. The mixture is stirred vigorously to obtain an even distribution of the discrete platinum particles within the molten lead. When only surface microelectrodes are desired, the particles are added, for example by sprinkling, to the surface of the liquid lead. After addition, the lead is allowed to cool until solid. The particles of platinum should then be uniformly small in size ranging from a fine powder to a dimension not above 50 mesh. The preferred material for microelectrodes is powdered, spongy platinum since it possess large surface areas and is relatively inexpensive when compared with platinum black or solid platinum metal. The more finely divided the platinum, the more efficient for the instant purpose. Because of economical considerations, the bielectrode should not contain more than one percent by weight of the precious metal. Satisfactory anodes have been prepared containing as little as 0.01% platinum, but the preferred amount for purposes of this invention is between 0.01% and 0.1% by weight.

As can be appreciated, bielectrodes of any desired size or shape can be made, for example by molding, rolling, hammering, etc. It should also be noted that the present article of manufacture is not limited to any specific shape or form so long as portions of the platinum metal are exposed to the electrode surface. It is, therefore, recomrnended that, prior to any coating operation, the surface of bielectrode be sanded to obtain a smooth surface and to expose more platinum particles on the electrodes surface. Prior to anodica'lly coating the electrode with lead peroxide, it is preferable that the electrode be pickled in acid. This pickling treatment minimizes any tendency of the lead peroxide to flake off during the anodic coating operation, and thus produces a more adherent and smoother coating of peroixed.

The electrolytic coating operation is preferably carried out by employing the bielectrode as an anode in the electrolysis of a chloride or sulfate solution. The concentration of the chloride ions can range from 0.1 normal to 1.3 normal; however, the preferred concentration is from 0.4 to 0.6 normal. In concentrations far below 0.1 or above 1.3 normal, it is diflicult to obtain a satisfactory coating of lead peroxide. Where the coating operation is carried out in a sulfate solution, it is preferred that the concent ation of sulfate ions be in a range above 0.05 normal. When a direct current is impressed across the electrolytic cell, a thin peroxide coating forms on the anodic surface. The more numerous the microelectrodes, the more rapid the formation of the coating. The thickness of the coating depends primarily on the current density and coating time. Anode current densities of 30-90 milliamps per square centimeter will produce the most satisfactory protective coatingstihe greater the current density, the more rapid the peroxide formation.

The following examples are illustrative of preferred modes of carrying out the invention and are not intended to be limiting.

Example 1 Commercial lead was heated to 340 in an iron ladle. When molten, 0.1% by weight of spongy platinum particles (which had passed through a U.S. Standard 200 mesh sieve) was added slowly while vigorously stirring the molten mass. Stirring was accomplished with a Waring Blendor, which dispersed the platinum particles into the bulk of the lead medium. During the stirring operation, the temperature of the mass was kept at 340 C. to prevent solidification. The molten lead, containing the dispersed, solid platinum particles, was then poured into a 12" x 12" flat square mold and allowed to cool and solidify. Then the cast bielectrode was sanded smooth and pickled in concentrated hydrochloric acid. The pretreated bielectrode was then made anodic in an electrolytic bath containing a 0.5 normal aqueous solution of NaCl. The cathode material was stainless steel 314. A direct potential was impressed across the electrode equal to a current density of 50 milliamps per square centimeter of anode area. Since chlorine gas was evolved at the anode and hydroxide at the cathode, it was necessary to add HCl to the electrolytic solution at various intervals to neutralize the hydroxide formed and to keep the concentrating of the chloride ions at -0.5 normal. After one hour of anodizing, the anode had developed a deep adherent coating of lead peroxide. The bielectrode, prepared and coated as above, was employed as an anode in an electrodialysis unit demineralizing 3,600 p.p.m. brackish water down to 300 p.p.m. Although the brackish water contained 2400 p.p.m. of dissolved NaOl, the anode revealed no appreciable corrosion or increase in voltage drop after four days of continuous operation.

Example 2 An electrode, employing antimonial lead as the base metal, was cast as in Example 1, but was not pickled in acid prior to being anodized in a 0.5 normal NaOl solution. During the electrolysis the peroxide coating formed, but a small degree of flaking occurred. The surface of this same anode was then sanded smooth (to remove any remaining oxide coating and to expose the platinum particles), pickled in concentrated HCl for 5 minutes, and employed once again as an anode. This time the film of lead peroxide formed smoothly and rapidly and no flaking was noticed. Thus, the preliminary surface treatment of sanding and pickling assists in producing a solid adherent coating of lead peroxide.

Example 3 A lead-palladium black bielectrode was constructed employing 0.01% by weight of the precious metal. The palladium was not dispersed through the molten lead but merely sprinkled on, and then scratched into, the lead surface. Upon cooling the article was lightly sanded (to surface expose more of the palladium particles), pickled in sur'furic acid, and anodically oxidized in a 1 normal solution of sodium sulfate at a current density of milliarnps per square centimeter of anode area. When a sufficient coating of peroxide had developed, the finished article was used as an anode in a caustic-chlorine cell operating on saturated brine solution. A current density of amps per square foot of anode area was employed,

and after 70 hours of operation, only slight anodic attack was visible.

What We claim is:

1. A composite article of manufacture for use as an anode eiectrode comprising a substantially fiat lead metal base having discrete particles of a spongy noble metal dispersed throughout said lead metal, the Working surface of said anode comprising an adherent layer of lead peroxide with said discrete particles 'being of a size no greater than 50 mesh and which comprises between 0.01% and 0.1% by Weight of said anode electrode.

2. The article of claim 1 wherein the discrete particles of the noble metal are platinum.

References Cited by the Examiner UNITED STATES PATENTS FOREIGN PATENTS 6/ 1961 Great Britain.

JOHN H. MACK, Primary Examiner. D. JORDAN, Assistant Examiner. 

1. A COMPOSITE ARTICLE OF MANUFACTURE FOR USE AS AN ANODE ELECTRODE COMPRISING A SUBSTANTIALLY FLAT LEAD METAL BASE HAVING DISCRETE PARTICLES OF A SPONGY NOBLE METAL DISPERSED THROUGHOUT SAID LEAD METAL, THE WORKING SURFACE OF SAID ANODE COMPRISING AN ADHERENT LAYER OF LEAD PEROXIDE WITH SAID DISCRETE PARTICLES BEING OF A SIZE NO GREATER THAN 50 MESH AND WHICH COMPRISES BETWEEN 0.01% AND 0.1% BY WEIGHT OF SAID ANODE ELECTRODE. 